Received12July2010,
Accepted26July2010
PublishedonlineinWileyOnlineLibrary:01September2010
(wileyonlinelibrary.com)DOI10.1002/bmc.1515
Arapidandsimpleassayforlamotrigineinserum/plasmabyHPLC,andcomparisonwithanimmunoassay
PhillipE.Morgan,*DanielleS.Fisher,RichardEversandRobertJ.Flanagan
ABSTRACT:Monitoringserum/plasmaconcentrationsoflamotriginemaybeusefulundercertaincircumstances.AnHPLCcolumnpackedwithstrongcation-exchange(SCX)-modifiedmicroparticulatesilicatogetherwitha100%methanoleluentcontaininganionicmodifierpermitsdirectinjectionofsampleextracts.AnHPLC-UVmethoddevelopedusingthisprincipleforthemeasurementofserum/plasmalamotrigineissimple,sensitiveandselective.Theanalysistimeislessthan5min.Intra-andinter-assayprecisionandaccuracymeetacceptancecriteria,andsamplestability,andpotentialinterferencesfromothercompoundshavebeenevaluated.Therewasgoodagreementwithconsensusmeanresultsfromexternalqualityassessmentsamples(n=32).Analysisofpatientsamples(n=115)usingtheHPLCmethodandtheSeradynQMS®Lamotrigineimmunoas-sayshowedthattheimmunoassayover-estimatedlamotrigineby21%onaverage.Copyright©2010JohnWiley&Sons,Ltd.Keywords:lamotrigine;strongcation-exchangeHPLC;non-aqueousioniceluents;immunoassay
Introduction
traceablepHreferencebuffersolutionswerefromMerck(Beeston,UK).CoarselyfilteredpooledhumanserumwasfromScipacLamotrigine(Fig.1)isusedinthetreatmentofepilepsy,either(Sittingbourne,UK).
aloneorincombinationwithotheranticonvulsants.ItisalsousedTrissolution(2mol/L,pH10.6)waspreparedbydissolving232gTrisinasamoodstabilizerinbipolardisorder,forexample.Lamotriginedeionizedwater(totalvolume2L),andthepHadjustedusingaqueousdoseadjustmentisnotstraightforwarddueto(i)largeinter-hydrochloricacid(6mol/L).Internalstandardsolution(5mg/Lnortrip-patientvariationsindoserequirementand(ii)importantinterac-tylinefree-base)waspreparedbydilutionofastocksolution(500mg/Ltionswithotheranticonvulsants,notablyphenobarbital,free-basein0.1mol/Lhydrochloricacid)withdeionizedwater,andstoredcarbamazepine,phenytoinandvalproicacid.Themeasurementat2–8°C.
ofserumorplasmalamotriginecanthereforebeuseful,particu-AstockHPLCeluentsolution(100mmol/L)waspreparedbyaddinglarlyduringdoseadjustmentandwhencombinationanticonvul-23.5gammoniumperchloratetoa2Lvolumetricflaskcontaining20mLsanttherapyisinitiatedorwithdrawn(Angelis-Stoforidisetal.,ofsodiumhydroxide(0.1mol/L)inmethanol,andmakinguptovolumewithmethanol.Thissolutionwasdilutedwithmethanoltogiveaneluent1999;Johannessenetal.,2003;Subramanianetal.,2008).
ionicstrengthof35mmol/Lammoniumperchlorate.TheeluentwasPublishedmethodsforserum/plasmalamotriginemeasure-vacuumfiltered(0.45mm,nylon66)andtheapparentpH(pH*)adjustedmentaregenerallybasedonreversed-phaseHPLC(RPLC)withUVto6.0usingperchloricacid(1%v/vinmethanol)beforeuse.Thecombi-detection(Angelis-Stoforidisetal.,1999;BarbosaandMídio,nationpHelectrode(Liq-Glass;Hamilton)wascalibratedusingaqueous2000;Becketal.,2006;Saracinoetal.,2007;Greiner-Sosankobuffers(pH4.00,7.00,and10.00).
etal.,2007),usuallyinvolvingevaporationofsampleextractsfol-lowedbyreconstitutionineluentpriortochromatographicMaterialsandequipment
analysis.Thestrongcation-exchange(SCX)HPLC-UVmethoddevelopedallowsdirectinjectionofsampleextractsandisGlasstesttubes(60¥7mm,Dreyertubes)werefromEsslab(Benfleet,UK).Sample/calibrationsolution/internalqualitycontrol(IQC)solutionssimple,fast,sensitiveandselective.Methodvalidationincorpo-werepipettedusingvariablevolumeair-displacementpipettes(BioHit,ratedacomparisonwiththeresultsobtainedfrompatientFinland),andinternalstandardaddedusingarepeatingvolumedispens-samplesusingtheSeradynQMS®lamotrigineimmunoassay.
ingpipette(Multipette®,Hamilton).Extendedfine-tippedpastetteswere
Experimental
Chemicalsandreagents
*Correspondenceto:P.E.Morgan,ToxicologyUnit,DepartmentofClinicalBiochemistry,King’sCollegeHospitalNHSFoundationTrust,DenmarkHill,LamotriginewasagiftfromGlaxoSmithKline(Stevenage,UK).SodiumLondonSE59RS,UK.E-mail:phillip.morgan@nhs.net
hydroxide(reagentgrade),tris(hydroxymethyl)aminomethane(Tris,ACSgrade)andnortriptylinehydrochloride(min98%)werefromSigmaToxicologyUnit,DepartmentofClinicalBiochemistry,King’sCollegeHospi-(Poole,UK).Ammoniumperchlorate(Ն98%)andperchloricacid(60%)talNHSFoundationTrust,DenmarkHill,LondonSE59RS,UK
werefromFluka(Poole,UK).Methanolandmethyltert-butylether(MTBE;Abbreviationsused:EQA,externalqualityassessment;IQC,internalqualitybothHPLCgrade)werefromRathburn(Walkerburn,UK),andNIST-control;MTBE,methyltert-butylether;SCX,strongcation-exchange.
Biomed.Chromatogr.2011;25:775–778Copyright©2010JohnWiley&Sons,Ltd.
775P.E.Morganetal.
ClClNH2NNResponse (mAU)40Lamotrigine (i) NNH2Nortriptyline20Figure1.Structuralformulaoflamotrigine.0usedtotransfersampleextractsinto0.5mLpolypropylenemicrotubeswithlids(Sarstedt,UK).Otherequipmentincludedvortexmixers,2mLpolypropylenescrew-toptubesandamicro-centrifuge.TheHPLCsystemconsistedofapump(3185PU),autosampler(3059AS),UV–visdetector(3075UV,220nm)andcolumnoven(CO2067;allJasco,GreatDunmow,UK).Theautosamplertraywasmaintainedat10°C.DataacquisitionandprocessingwereperformedusingEZChromElite(Agilent,version3.2.1).Astainless-steelHPLCcolumn(100¥2.1mmi.d.)packedwithWatersSpherisorbS5SCX(propylsulfonicacid-modified)silica(HiChrom,UK)wasusedwithastainless-steelguardcolumn(10¥2.1mmi.d.)packedwiththesamematerial.Thecolumnoventemperaturewasmaintainedat35°C.Theeluentflow-ratewas0.5mL/min,andtheinjec-tionvolumewas40mL.Differenceplotsandt-testresultswereproducedusingAnalyse-it(version2.21)withinMicrosoftExcel.0123Time(min)(ii) 440Nortriptyline Response (mAU)Lamotrigine 200PreparationofcalibrationandinternalqualitycontrolsolutionsStocksolutions(1.00g/Llamotrigine)werepreparedbyweighing50.0mglamotrigineintovolumetricflasks(50mL)andmakinguptovolumewithhydrochloricacid(0.1mol/L).Calibrationsolutionswerepre-paredbydilutionofastocksolutionwithpooledhumanserumtogivesolutionscontaining0.5,2.0,5.0,10.0,20.0and30.0mg/Llamotrigine.IQCsolutionscontaining1.0,8.0and25.0mg/Llamotrigineweresimilarlypreparedinpooledhumanserumfromaseparatestocksolution.Afterthoroughmixingandequilibration(24h,2–8°C),thesolutionsweredividedintoapproximately200mLportionsandstoredin2mLpolypro-pylenetubesat-20°Cuntilrequired.0123Time (min)4(iii)Response (mAU)321SamplepreparationSample,calibrationsolutionorIQCsolution(50mL),internalstandard(50mL)andTrissolution(50mL)werevortex-mixed(5s)inaDreyertube.MTBE(200mL)wasadded,andthetubecontentsvortex-mixed(20s)andcentrifuged(11,000g,4min).Aportion(>60mL)oftheMTBEextract(upperlayer)wasaddedtoamicrotube,andthetubewascapped.00123Time 4Figure2.Chromatogramsofserumextractsfrom(i)internalqualitycontrolsolution(8mg/Llamotrigine),(ii)patientsample(prescribeddoseoflamotrigine200mg/day,concentrationoflamotrigine4.1mg/L),and(iii)analyte-freeserumwithoutadditionofinternalstandard(arrowsindi-catetheretentiontimesoflamotrigineandnortriptyline).HPLCmethodvalidationThepeakarearatiooflamotriginetotheinternalstandardwasplottedagainstconcentrationoflamotrigineinthecalibrationstandards,andalinefittedbyleastsquaresregression.CalibrationstandardsandIQCsolu-tionswereassayedatthebeginningandendofeachanalyticalsequence,withIQCsalsorepeatedafterevery10injections.Patientsampleswereanalysedinduplicate.Assayacceptancecriteriawere(i)correlationcoefficient>0.98,and(ii)IQCvalueswithinϮ15%oftheirnominalvalue.Intra-andinter-assayprecision(%RSD)andaccuracyweremeasuredbyreplicateanalysis(n=10)oftheIQCsolutionsonthesameday,andduplicateanalyses(meanofduplicates)ondifferentdays(n=5),respec-tively.ThestabilityoflamotrigineinserumwasevaluatedbyanalysisofIQCsamples(i)throughthreefreeze–thawcycles,theassaybeingcali-bratedeachtimewithstandardsolutionsthathadnotbeenthawedpreviously,and(ii)beforeandafterstandingfor24hatroomtempera-ture.RecoveryoflamotriginewasinvestigatedbycomparisonofthemeanpeakareaoflamotriginefromextractedsolutionsofeachlevelofIQC(n=3)withthemeanpeakareaobtainedfromnon-extractedsolu-tionsattheequivalentconcentration(n=10).Serumsamples(n=32)containinglamotriginefromtheUKNEQASHeathcontrolTDMexternalqualityassessment(EQA)scheme(CardiffBioanalyticalServices,UK)wereassayedretrospectively,butwithoutpriorknowledgeoftheconsensusmeanresults.ResultsTypicalchromatogramsareshowninFig.2.Thecalibrationgraphwaslinearovertherangeused.Injection(2mL)ofseparatesolu-tionscontainingleviteracetam,oxcarbazepine,vigabatrinandrufinamide(all10mg/Linmethanol)showedthatthesecom-poundswereunretained.Furthermore,thepresenceofother776wileyonlinelibrary.com/journal/bmcCopyright©2010JohnWiley&Sons,Ltd.Biomed.Chromatogr.2011;25:775–778
LamotrigineassaybySCX-HPLC
sd90% ohtem80% hto70% b eIdentitym con60% refr etflfui50% Bias (20.5%)sde trn ne40% 95% Limits of agreementacerMep30% (-8.2% to 49.1%) / s)Ca20% LPH 10% - SMQ0% (-10% 0 5 10 152025Mean result from both methods (mg/L)Figure3.Differenceplot:QMS®LamotrigineassayandHPLCassay,samplesfrompatientsprescribedlamotrigine(n=115).anticonvulsantsintheEQAmaterialdidnotinterferewitheitherlamotrigineornortriptyline.Outofarangeofpsychoactivecom-Discussionpoundsalsotestedundertheseconditions,desipramine,norclo-Apreviousstudy(WestleyandMorris,2008)reportedasignifi-mipramineandprotriptylinemayinterferewithnortriptyline,andcantoverestimation(6%)oflamotrigineconcentrationsinfluvoxaminewithlamotrigine.patientsamplesmeasuredusingtheSeradynQMS®immunoas-Inter-andintra-assayprecisions(RSD)wereintheranges3.1–saycomparedwithHPLC.Inourstudy,thisbiaswas21%.While4.1and4.2–4.4%,respectively,withaccuracybetween101andWestleyandMorrisusedplasma,ourpatientsampleswere103%ofnominalconcentrations.Recoveryoflamotriginefromserum,whichmayaccountforsomeofthedifference.However,humanserumwas81Ϯ3%.NosignificantlossoflamotriginethemanufacturerstatesthattheQMS®Lamotrigineassayissuit-fromserumwasobservedduringthestabilitystudies.Resultsableforeitherserumorplasma.TheobservationthattheQMS®fromthere-analysisofcalibrationstandardsandIQCsolutionsimmunoassayshowednosignificantbiaswiththeEQAsamplesafterstoragefor14monthsat-20°C(assaycalibratedusingstan-comparedwiththeHPLCmethoddevelopedsuggeststhattheredardsolutionspreparedfromfreshlypreparedstocksolutions)maybecrossreactionwithlamotriginemetabolitesorothercom-gavemeasuredlamotrigineconcentrationswithinϮ16%ofponentspresentinpatientsamples,andthatfurtherinvestiga-nominal.Thelimitofaccuratemeasurement,takenasapeakof5tionmaybenecessary.timestheaverageheightofthestandarddeviationofthebase-TheadvantagesofusingSCX-modifiedmicroparticulatesilicalinenoise,was0.1mg/L.HPLCpackingsinbasicdruganalyseshavebeendocumentedEQAsampleswereassayedinduplicate.Samplesthatdidnot(Flanaganetal.,2001;Morganetal.,2010).Despitebeingaweakcontainlamotrigine(n=3)wereidentifiedassuchbytheHPLCbase(pKa5.5–5.7),lamotrigineisretainedontheSCXcolumnmethod.Results(n=29,excludingthreenotcontaininglamot-undertheconditionsdescribed.TheinherentselectivityofSCXrigine)showedgoodagreementwithconsensusmeanresults.forprotonatedanalytesresultsinnointerferencefromotheranti-Comparisonusinglinearregressiongave:slope(95%CI)=1.03convulsants,offeringarealalternativeforlamotrigineassay.On(1.01–1.05);y-axisintercept(95%CI)=0.04(-0.06to0.14);theotherhand,thismethodisnotsuitableifotheranticonvul-R2=0.998.santsneedtobemeasuredatthesametimeaslamotrigine.Thepotentialforinterferencefromdesipramine,norclomipramine,protriptylineandfluvoxamine,andindeedfromnortriptyline,isComparisonwithimmunoassayverylowinpracticeevenifco-prescribedwithlamotrigine,sinceTheHPLCmethoddescribedwasusedtocompareresultsfromtheconcentrationsofthesecompoundsachievedintherapyareserumsamplesfromanEQAscheme(UKNEQASHeathcontrolrelativelylow.TDM,n=32)andfrompatientsprescribedlamotrigine(n=115)withtheSeradynQMS®immunoassay.NosignificantdifferenceConclusionswasobservedfortheEQAsamples(pairedt-test,t=-0.51).Wehavedevelopedandvalidatedafast,straightforwardHPLCHowever,forthepatientserumsamplestheimmunoassayresultsmethodforserum/plasmalamotriginemeasurement.Theweresome20.5Ϯ1.36%higherthanthoseobtainedbyHPLCmethodisselective,accurateandallowsdirectinjectionof(pairedt-test,t=-11.45,d.f.=114,p=<0.001;Fig.3).Measure-sampleextracts.ComparisonofresultsfrompatientsampleswithmentoftheserumcalibratorsprovidedwiththeimmunoassaykitthosefromtheSeradynQMS®Lamotrigineimmunoassayindi-byHPLCgavethefollowingresults(stated/foundconcentration,catedthattheimmunoassayshouldbeusedwithcautionuntilmg/L):2.0/2.4;5.0/5.0;10.0/9.4;20.0/19.2;40.0/38.9.furtherevaluations/investigationshavebeencompleted.Biomed.Chromatogr.2011;25:775–778Copyright©2010JohnWiley&Sons,Ltd.wileyonlinelibrary.com/journal/bmc
777P.E.Morganetal.
Acknowledgements
WethankProfessorPhilipPatsalosforhelpfulcomments.
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